#include<bits/stdc++.h>
using namespace std;


#define ll long long int


int main(){
    int x,y,z;
    cin>>x>>y>>z;
    int diff=0;
    if(x-y == x-z) diff=x;
    else if(x-y == y-z) diff=y;
    else if(x-z == y-z) diff=z;
    
    if(diff<0) diff*=-1;
    cout<<diff;
}